Known autofocus devices comprise an autofocusing optic for generating an autofocus beam path that generates an autofocus pattern on the object, the autofocus beam path usually being guided onto the object by means of a deflection element. An autofocus device of this kind furthermore comprises an autofocus evaluation unit to which a detector beam path is delivered by means of a deflection element, said detector beam path representing the radiation, reflected from the object, of the autofocus beam path.
DE 41 31 737 C2 describes an autofocus device for a stereomicroscope having a main objective. With this autofocus device, a projection beam path is generated by a projection optic and in turn, after passage through the main objective, generates a line pattern on the object surface. Light of the projection beam path reflected from the object is coupled out and is delivered to a light-sensitive, spatially resolving position detector. A defocusing operation results in a lateral shift of the image of the line pattern (linear marking) on the position detector, this lateral shift being recorded via the autofocus evaluation unit and being used as a closed-loop control signal for refocusing for a focus drive. For autofocusing, the latter adjusts the intercept distance of the objective being used, or displaces the entire stereomicroscope along the optical axis.
DE 10 2006 040 636 B3 proposes, based on the arrangement described in accordance with DE 41 31 737 C2, an autofocus device in which a micromirror array having individually controllable and adjustable micromirrors is provided in shared fashion as a deflection element for the autofocus beam path and the projection beam path, and as a deflection element for the detector beam path. By means of the micromirror array, the autofocus pattern can be modified in controlled fashion both in terms of its location and geometry, and in terms of its light intensity, by corresponding application of control to the micromirrors. In addition, the illumination optic that is in any case present in a microscope for illuminating the object can be used to generate the autofocus beam path. The micromirror array can thus take on two functions: on the one hand it generates, from the illumination beam path, the autofocus beam path for generating an autofocus pattern on the object; and on the other hand it couples out the detector beam path, which in turn is guided onto the autofocus evaluation unit. This document describes the generation of two spot beams by suitable adjustment of micromirrors in the micromirror array. The spot beams proceeding from the micromirror array pass through the main objective of the microscope and are focused by it. If the intersection point of the two spot beams is located above or below the object, two spots are then visible on the object. For autofocusing, a main objective of fixed focal length is shifted until only one spot (merger of the two spots visible when defocused) is visible on the object. With a vario objective (objective of variable focal length), the focal length is adjusted accordingly. Regarding details of this and other types of autofocusing using the autofocus device described, reference is made explicitly to the aforesaid patent document DE 10 2006 040 636 B3. By this reference, the corresponding disclosure is expressly intended to become a constituent of this Application.
Another autofocus device is known from EP 1 241 506 B1. Here a (video) camera, which is present on a surgical microscope for documentation purposes, is used to locate the correct focus position of the microscope. The camera comprises an image acquisition device and a signal processor unit, the latter ascertaining the optimal focus using, for example, the contrast method. By means of an interface in the camera and a further interface in the microscope, the closed-loop control signals generated by the camera are transferred, for focusing, into a control unit of the microscope, which unit applies control to corresponding positioning units in the microscope which modify the objective focal length for autofocusing. Except for the aforesaid (video) camera, a separate measurement module is not used with this system.
Autofocusing is performed in similar fashion in EP 1 255 146 B1; here a two-dimensional histogram is produced by evaluating the change in the image content of a microscope image sensor, and from that histogram a specific motion activity is derived. An autofocus unit can thereby, based on the data ascertained, place the focus into a region of maximum activity, i.e. for example into a region investigated with a surgical instrument.
DE 33 28 821 C2 discloses an autofocus device for microscopes having at least two alternatingly switched light sources, such as infrared diodes, which are arranged next to one another in the vicinity of a pupil of the microscope. The beams proceeding from the light sources are reflected into the beam path of the microscope. The light sources illuminate a mark that represents, in particular, a grid constituted from transparent and reflective stripes. This mark is imaged in autocollimation onto itself If the object is located at the focus of the microscope, the beams proceeding toward the object through the transparent stripes of the mark once again proceed, on the return path, through the same regions of the mark. If the object is not in focus, however, a beam proceeding through a transparent stripe of the mark then experiences a lateral offset upon reflection at the object, and arrives onto a reflective strip arranged above or below the transparent stripe (depending on the sign of the defocusing). If the reflective stripes are arranged at a specific angle to one another adjacently to the transparent stripes, the sign of the defocusing can then also be ascertained.
Lastly, an autofocus device referred to as “SpeedSpot,” which works with two laser beams that generate two spot-shaped markers on the object which in turn merge into one spot when the focus position of the object plane is optimal, is known from the Leica model M720 OH5 surgical microscope. The customer selects a fixed focusing speed for autofocusing. Focusing occurs in such a way that the laser spots that are spaced apart are merged into one spot by bringing the object plane closer to the focal plane. If there is a large deviation between the optimum focal plane and the object plane, the procedure of bringing the object plane closer to the optimum focal plane is usually perceived by the user to be too time-consuming. At high magnifications, conversely, the fixed focusing speed can cause the region of the focal plane to be passed through too quickly. Because of the shallow depth of field at high magnifications, detection of the merger of the laser dots, in particular by a user, is often possible only with difficulty.
The object that arises in the context of the present invention is therefore that of improving an autofocusing method, based on the use of, in particular, spot-shaped markers, in such a way that simple, exact, and rapid autofocusing is possible regardless of the microscope magnification that is set.